Chroma overload circuits are commonly used to prevent oversaturated chroma which may result from "channel impairments" in a video system such as tuner tilt, multipath propagation effects, noise or the like.
A conventional form of chroma overload protection circuit operates by adjusting the gain of the chroma signal in response to an average chroma level during active video time intervals after automatic chroma control (ACC) processing. FIG. 1 illustrates an example of such a system. The conventional system of FIG. 1 comprises series connected ACC unit 10 and chroma overload unit 20 (each outlined in phantom) followed by chroma signal separator circuit 30 which separates the chroma signal into its component vector components, U and V for further processing (e.g., demodulation to baseband and matrixing to provide baseband color difference output signals R-Y and B-Y).
ACC unit 10 comprises controllable gain amplifier 12 to which a chroma input signal is applied and a feedback path from the output of amplifier 12 to control input 14 thereof. The feedback path comprises a series connection of burst gate 16 and automatic chroma control circuit (ACC) 18. Chroma overload circuit 20 is also feedback controlled and comprises controllable gain amplifier 22 which is coupled at the input thereof to receive the ACC controlled chroma from unit 10 and is provided with feedback to control input 24 thereof via a series connection of gate 26 and average detector 28. Chroma signal separation into its component vectors is performed after chroma ACC and chroma overload protection by chroma signal separator circuit 30 connected to the output of overload circuit 20 to provide separated chroma vector component output signals U and V.
In the operation of ACC unit 10, burst gate 16 passes the color burst component of the chroma signal to ACC unit 18 which compares the burst amplitude with a reference level and provides a control signal to control input 14 of amplifier 12 to adjust the chroma signal amplitude to a predetermined level. In this way, the amplitude of the overall chroma signal C is stabilized at a predictable value based on the burst amplitude relative to the reference or desired level. If the burst amplitude increases, the gain of amplifier 12 decreases to thereby stabilize the average chroma signal level. Some smoothing may be included in ACC unit 18 to prevent noise which may be present on the gated burst signal from disturbing the regulated chroma output signal level.
In the operation of chroma overload circuit 20, gate 26 is open during the active video trace interval and closed otherwise to thereby pass only chroma to average detector 28. Recall that ACC detector 18 serves to regulate the chroma level based on the gated burst amplitude. Overload detector 28 serves to provide chroma overload limiting based on the average chroma level (rather than the burst level). For this purpose, the time constant of average detector 28 as well as the gain characteristic of gain controlled amplifier 22 determine the behavior of the overload circuit. In some systems of this type, the attack and decay time constants of detector 28 may be different, and detector 28 may be implemented as a so-called "leaky peak detector" (e.g., a capacitor with a fast charging circuit and a parallel connected "leak" resistor to provide a relatively slow discharge time constant).